The present invention relates to a method of controlling an ion implanter in plasma immersion mode.
The field of the invention is that of ion implanters operating in plasma immersion mode. Thus, implanting ions in a substrate consists in immersing the substrate in a plasma and in biasing it with a negative voltage of a few tens of volts to a few tens of kilovolts (generally less than 100 kV), so as to create an electric field capable of accelerating the ions of the plasma towards the substrate so that they become implanted therein. The atoms implanted in this way are referred to as “dopants”.
The penetration depth of the ions is determined by their acceleration energy. It depends firstly on the voltage applied to the substrate and secondly on the respective natures of the ions and of the substrate. The concentration of implanted atoms depends on the dose, which is expressed as a number of ions per square centimeter, and on the implantation depth.
For reasons associated with the physics of plasmas, a few nanoseconds after the voltage has been applied, a sheath of ions is created around the substrate. The potential difference responsible for accelerating the ions towards the substrate lies across the boundaries of this sheath.
The growth of this sheath as a function of time follows the Child-Langmuir equation:
      j    c    =            4      9        ⁢                            ɛ          0                ⁡                  (                                    2              ⁢                                                          ⁢              e                        M                    )                            1        /        2              ⁢                  V        0                  3          /          2                            S        2            where:                jc is current density;        ε0 is vacuum permittivity;        e is the charge of the ion;        M is the mass of the ion;        V0 is the potential difference across the sheath; and        s is the thickness of the sheath.        
By stipulating that the current density is equal to the amount of charge passing through the boundary of the sheath per unit time, ds/dt represents the travel speed of this boundary:
            ⅆ      s              ⅆ      t        =            2      9        ⁢                            S          0          2                ·                  u          0                            S        2            
In which expression, s0 is given by:
      s    0    =            (                        2          ⁢                      ɛ            0                    ⁢                      V            0                                    e          ·                      n            0                              )              1      /      2      it being understood that u0=(2 eV0/M) is the characteristic speed of the ion and that n0 is the density of the plasma.
The thickness of the sheath is associated mainly with the applied voltage, with the density of the plasma, and with the mass of the ions.
The equivalent impedance of the plasma, which conditions the implantation current, is directly proportional to the square of the thickness of the sheath. The implantation current thus decreases very quickly as the sheath becomes thicker.
After a certain length of time, it is necessary to proceed with re-initialization. This becomes practically essential once the sheath reaches the walls of the enclosure, thereby stopping the implantation mechanism.
In order to reinitialize the system, nearly all implanter manufacturers switch off the high voltage on the substrate, while maintaining the plasma ignited. It is therefore necessary to have a pulse generator that produces high-voltage pulses.
Thus, with reference to FIG. 1, document WO01/15200 proposes biasing the substrate by means of a power supply that comprises:                a generator GEN having its positive pole connected to ground;        a capacitor Ct in parallel with the generator GEN;        a first switch IT1 having its first pole connected to the negative pole of the generator GEN and having its second pole connected to the output terminal O of the power supply; and        a second switch IT2 having its first pole connected to the output terminal O and having its second pole connected to ground.        
The method comprises the following stages:                an implantation stage during which:                    the plasma power supply is activated;            the first switch IT1 is closed; and            the second switch IT2 is opened;                        a neutralization stage during which:                    the first switch IT1 is opened; and            the second switch IT2 is closed.                        
The continuous presence of the plasma inside the enclosure gives rise to undesirable side-effects:                particles are generated;        heat is delivered to the substrate;        the enclosure is attacked, giving rise to risks of that parts being processed will suffer metal contamination;        charge effects are created, which effects are particularly troublesome in microelectronics applications; and        during the stages in which the voltage applied to the substrate is rising and falling, implantation takes place at an acceleration voltage that is not stabilized.        
Furthermore, document US 2007/069157 provides for the following succession of operations:                activating the substrate power supply;        after a certain time delay, activating the plasma power supply for the duration of one pulse;        deactivating the plasma power supply; and        after a certain time period, deactivating the substrate power supply.        
It follows that during said period, the substrate power supply is activated and the plasma power supply is deactivated, which corresponds to a relaxation stage.
During the implantation stage, the zones of the substrate and that are electrically insulating become positively charged, with this taking place in cumulative manner. It goes without saying that this situation is not desirable, if only because of the resulting disturbances to the implantation process. It is therefore desirable to neutralize these positive charges by supplying electrons.
It is thus possible to provide a filament, however a filament will tend to vaporize. It is also possible to provide an electron gun, but that constitutes additional equipment that is relatively burdensome.